Polyarylene sulfides (also may be abbreviated as “PAS” hereafter) such as polyphenylene sulfides (also may be abbreviated as “PPS” hereafter) are engineering plastics having excellent heat resistance, chemical resistance, flame retardant properties, mechanical strength, electrical properties, dimensional stability, and the like. PAS is widely used in a range of technical fields including those of electrical instruments, electronic instruments, automobile instruments and packaging materials, because PAS can be formed into various molded products, films, sheet, fibers, and the like by general melting processing methods such as extrusion molding, injection molding and compression molding.
A typical manufacturing method for PAS includes performing a polymerization reaction of a dihalo aromatic compound (hereinafter sometimes referred to as “DHA”) such as paradichlorobenzene (hereinafter sometimes referred to as “pDCB”) with a sulfur compound such as alkali metal sulfide or alkali metal hydrosulfide as the sulfur source, in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter sometimes referred to as “NMP”) under heated condition to obtain a PAS-containing reaction solution, subsequently separating PAS from the PAS-containing reaction solution and recovering PAS by washing and drying.
This polymerization reaction is a desalting polycondensation reaction, wherein, in addition to the reactant PAS, by-product alkali metal salts such as alkali metal halides (e.g. NaCl), low polymers such as dimers, and trimers, and impurities (volatile substances and substances with a high boiling point, and the like) are also produced. For this reason, these organic amide solvents, byproduct alkali metal salts, low polymers, and impurities may be present either between or inside PAS particles, or in the reaction solution after the polymerization reaction. Accordingly, PAS separated from the PAS-containing reaction solution is recovered after washing thoroughly to remove the organic amide solvent, byproduct alkali metal salts, low polymers, and impurities, thereby improving and maintaining the quality of the PAS used as product.
At the same time, the separation liquid from which PAS was separated via solid-liquid separation of the PAS-containing reaction solution, contains microscopic particulate form PAS (hereinafter sometimes referred to as “raw material fine PAS powder”). However, this raw material fine PAS powder is not as good as the PAS product from a quality perspective (molecular weight, color, smell, gas generation, and the like), and as a result, it is not recovered as a product, but is disposed of. In order to comply with environmental criteria during disposal, raw material fine PAS powder is disposed at present as follows: raw material fine PAS powder is recovered from the separation liquid by solid-liquid separation using filtration, and the like, and the organic amide solvent, byproduct alkali metal salts, low polymers, and impurities are removed from between and in the fine particles of raw material fine PAS powder by washing, then the raw material fine PAS powder is disposed after confirming compliance with environmental criteria (for example, landfill or incineration).
Furthermore, even if raw material fine PAS powder is offered as a product, it has little value in industrial use, and causes few problems upon disposal, due to its small scale of production (hereinafter, where raw material fine PAS powder is recovered and offered as a product, the quantity of the product is sometimes referred to as the “productization yield”).
However, around 30 years have now passed since PAS first entered the market, and along with demands for quality, the market has also come to demand cost reductions, and these demands have been increasing each year. For that reason, there has been a general review of the steps by which PAS is manufactured.
Against this background, from the perspectives of reducing PAS costs and responding to environmental problems, studies for recovery of raw material fine PAS powder as a product, which has been recovered from the separation liquid and disposed of in the past, has been conduced.
In Patent Document 1, specifically, a method is proposed in which, particulate polymer is separated using a 60 mesh screen, after polymerization for 3.0 hours at a reaction temperature of 260° C., and a PAS oligomer is coagulated by adding water to the mixture containing PAS oligomer and solvent after removal of NaCl from the separation liquid, then the PAS oligomer is separated by centrifugal separation.
In this case, 60 mesh has an aperture of 250 μm, so oligomer with a particle size of 250 μm or smaller is selected. In other words, in Patent Document 1, possibly as a result of the polymerization method, PAS polymer with a particle size of 250 μm or greater is selected as a product, and separated from PAS oligomer with a particle size of 250 μm or smaller.
In Patent Document 2, a method is proposed in which, using a phase separation agent, a slurry containing granular PAS, PAS oligomer, organic polar solvent, water, and halogenated alkali metal salt is obtained via polymerization, and the PAS oligomer is separated from the slurry. Specifically, an 80 mesh (175 μm) sieve is used to separate granular PAS, after which a glass filter of aperture 10 to 16 μm is used to separate the PAS oligomer. In this case, the PAS oligomer selected has a distribution ranging from a minimum particle size of from 10 to 16 μm to a maximum particle size of 175 μm.
In Patent Document 3, a manufacturing method for PAS resin is proposed wherein the PAS oligomer obtained using the method in Patent Document 2 is subjected to thermal oxidation in an oxidizing gas atmosphere at from 150 to 260° C., in order to reduce volatile substances.
However, these citations do not specifically disclose the problems associated with recovery of raw material fine PAS powder for use as product from separation liquid, or any problems with quality when compared to that of a regular product.